Postmenopausal estrogen replacement therapy (ERT) is associated with a lower incidence of cardiovascular disease in women, especially in those with established coronary artery disease. This study seeks to determine if ERT with or without low dose progesterone will slow the progression of or induce regression of the anatomic and functional sequelae of atherosclerosis on coronary arteries in postmenopausal women with established coronary atherosclerosis.